Triangles or Squares or both? Which polys are easier for to render and in number? Or what if there are more verts than 3 or 4?
As far as I know, fundamentally speaking computers only understand triangles, nothing more.

So technically whatever you use will be converted to a three sided polygons only representation of the model at render time regardless. Having a triangle based mesh or a quad based mesh should not make a very significant difference at render time.

Storing a triagulated version of a mesh however will require more data and hence more memory and storage, because of the additional edge info that is being recorded. It should not be a very significant difference though, unless you have a very large and complex mesh.


Blue is quad based Suzane, orange is the same mesh triangulated

What you should be concerned with instead is good topology to make your models easy to work with, easy to edit or modify.

Quad based topologies are also good for animation, by being easier to rig and better at deformation based transforms, it makes them easier to texture through UV Unwrapping too.